1. Field of the Invention
The present invention relates generally to a system for controlling operation of an internal combustion engine, such as fuel supply amount, spark ignition timing and so forth. More specifically, the invention relates to an engine control system which can provide an improved control characteristics at a transition state of an engine driving condition.
2. Description of the Background Art
It is conventionally known typical engine control to monitor an intake air related parameter representative of an intake air amount and an engine speed releted parameter representative of an engine revolution speed. Based on the intake air related parameter and the engine speed related parameter, a basic fuel supply amount T.sub.p is derived. The basic fuel supply amount T.sub.p is corrected by a correction value which is derived on the basis of various correction parameters, such as an engine coolant temperature and so forth. The corrected value is output as a final fuel supply data Ti. On the other hand, a spark ignition timing id determined on the basis of the basic fuel supply amount T.sub.p and the engine speed.
In order to monitor the intake air related parameter, an air flow meter or an intake air pressure sensor has been used. Because of lag of such air flow meter or intake air pressure sensor, the intake air related parameter varies to increase and decrease following to actual variation of the intakle air flow amount with a certain lag time. In case of acceleration, such lag of response in variation of the intake air related parameter results in leaner mixture to raise emission problem by increasing of amount of NO.sub.x and HC. Furthermore, due to lag in variation of average effective pressure, acceleration shock and degradation of engine acceleration characteristics can be caused. In addition, since the fuel supply amount becomes smaller than that required. spark advance tends to be excessively advanced to cause engine knocking.
In order to improve such drawback caused by lag of the air flow meter or the intake air pressure sensor, Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-201035 discloses a technique for correcting the intake air flow rate measured by the air flow meter or the intake air pressure measured by the intake air pressure sensor according to a variation ratio of a throttle valve open angle in order to derive an assumed intake air flow rate or an assumed intake air pressure. In such system, since the fuel supply amount is derived on the basis of the corrected intake air related parameter, i.e. intake air flow rate or intake air pressure, fluctuation of air/fuel ratio can be minimized for better transition characteristics.
However, because the intake air flow rate and the intake air pressure do not correspond linearly to the throttle valve open angle, expensive work has been required for determining correction values for respective throttle valve angular positions. By extensive work for setting the correction values, cost for the control unit becomes high. Furthermore, though the proposal in the aforementioned Japanese Patent First Publication No. 60-201035 improves response characteristics, it cannot achieve satisfactorily high precision level because the disclosed system does not concern about difference of timing between a timing of measurement of the intake air flow rate or the intake air pressure and a timing of variation of the throttle valve angular position.